Sucrose esters are widely recognized as having insecticidal properties in plants and have shown toxicity toward soft-bodied insects, including aphids, mites, pear psylla, and whiteflies, as well as antibiotic activity. Sucrose esters are also considered as flavour precursors and are accumulated on the leaf surface during the life of the plant. They are stable compounds that can be identified in cured and smoking material. Esterified small carboxylic acids are released from sucrose esters upon heating. These small carboxylic acids are very potent flavour molecules and are considered to be responsible in part for the oriental flavour of tobacco. Sucrose esters are produced in the glandular trichome cells of plants. The glandular trichome cells are also the place for synthesis of other leaf exudates—such as proteins (phylloplanins) and diterpenoids (cembrenoids and labdenoids). The sucrose and the small carboxylic acids that are esterified to the sucrose to give rise to the sucrose esters are produced via two distinct metabolic blocks. Tobacco varieties differ in their quantities and qualities of the sucrose ester present on the surface of the leaf. As a general rule, flue cured, burley and Maryland varieties accumulate low amounts of sucrose esters with acyl groups up to five carbon chain length whereas most oriental varieties and many cigar tobacco types accumulate high amounts of sucrose esters having acyl groups up to six carbon chain length, and to a lesser extent, esters having acyl groups with a seven carbon chain length can be observed. The dichotomy observed in the sucrose ester produced among tobacco varieties is linked to a single dominant locus called BMVSE (beta-methylvaleryl containing sucrose esters) for its functional allele and bmvse for its non functional allele. The BMVSE genomic locus has been positioned to chromosome A of tobacco.
Kroumova and Wagner (2009) General and Applied Plant Physiology 35, 3-4, p 95-110 describe attempts to change sucrose ester acyl group content in various plants using a reverse genetics approach to knock down the expression of isopropylmalate synthase expression using double stranded interfering RNA. In this study, the isopropylmalate synthase gene from Solanum pennellii was used to amplify part of the genes from cDNA. They were then introduced in the sense and antisense orientation into a double stranded interfering RNA construct and transformed via Agrobacterium into Nicotiana tabacum T.I. 1068, Nicotiana glutinosa cv. 24a and S. pennellii. The results obtained using N. tabacum plants showed some changes in the acyl group abundance of sugar esters through reduced beta-methlyvaleryl and increased 2-methylbutyryl-acylation compared to non-transformed controls. However, the N. tabacum plants that were obtained were phenotypically impaired since they were chlorotic and some had curly leaves. Likewise, the transformed N. glutinosa and S. pennellii plants were phenotypically impaired. The authors conclude that isopropylmalate synthase is a critical enzyme and that its impairment probably would lead to an unhealthy plant.
There is a need in the art for plants in which the sucrose ester composition is modulated and whilst minimising undesirable effects on the plant. It is an object of the present invention to satisfy this need.